REPORT OF THE DIRECTOR T9IO 1 93 



Mohawk valley, some of which have been definitely traced into 

 the Precambric area which lies between these regions. As shown 

 on the accompanying maps, this faulting is important for our pres- 

 ent consideration because the patchwork effect of the igneous and 

 Grenville rocks has often been either produced or sharply accentu- 

 ated by this means and many of the finest exhibitions of exfoliation 

 are on the fault-scarp sides of the domes. Huckleberry, Crane, and 

 Little mountains would doubtless not be separated by the narrow 

 Grenville belts except for this faulting. Also it should be stated 

 that a fault almost certainly extends along the western base of 

 Potash mountain with the Grenville sharply faulted against the 

 base of the great dome. 



3 Glaciation. It has already been shown by the writer^ that 

 this region has been subjected to vigorous glaciation, especially the 

 southern portion of the North Creek sheet. Before the Ice Age 

 the lowlands must have been covered with much residual soil while 

 the mountains bore great accumulations of talus material on their 

 sides and especially at their bases. The advancing ice almost com- 

 pletely removed these materials but, more than this, there is strong 

 evidence that, by ice erosion, the Grenville valleys of weak rocks 

 were considerably deepened. If so, the mountains of resistant rock 

 were doubtless bared and rounded off. Except in a few cases of 

 valleys transverse to the direction of ice movement, the outcrops 

 of Grenville and igneous rocks alike are hard and fresh. Most of 

 the loose material now occupying the lowlands is glacial debris of 

 lake or morainic origin. As a result of glaciation the mountains 

 were completely bared of weathered material and vegetation; were 

 often increased in height above the surrounding country; and the 

 fault-scarps were often accentuated in steepness. Thus the pre- 

 glacial igneous masses were left in a very favorable condition for 

 postglacial exfoliation which is now so prominent. The interesting 

 fact that there is now no great accumulation of talus material 

 around the bases of the mountains is thus readily accounted for. 



4 Temperature changes, humidity etc. Chamberlin and Salis- 

 bury^ state that " the breaking of rock by changes of temperature 

 should be greatest on the bare slopes of isolated elevations of crys- 

 talline rock, where the temperature conditions of temperate latitudes 

 prevail, and where the atmosphere is relatively free from moisture. 

 All of these conditions are not often found in one place, but the 



1 Paper read before the Pittsburg (1910) meeting of the Geological Society 

 of America. 



^ Geology, i :49. .. - 



